A cellular communications system projects any number of cells over the earth at diverse locations. A frequency spectrum is then allocated in frequency, in time, by coding, or a combination of these, to the cells so that communications taking place in nearby cells use different channels to minimize the chances of interference. On the other hand, communications taking place in cells located far apart may use the same channels, and the large distance between communications in common channels prevents interference. Over a large pattern of cells, a frequency spectrum is reused as much as possible by distributing common channels over the entire pattern so that only far apart cells reuse the same spectrum. An efficient use of spectrum results without interference.
One problem which cellular communications systems address is the handing off of communications between cells. Relative movement between end users and cells causes the end users and the communication links directed thereto to move between cells. In order to permit continuous communications in an ongoing call, the system must "handoff" the call when the end user crosses a cell boundary. If a call is not handed off to a new cell upon leaving an old cell, the call will eventually be lost because the strength of signals over which communications take place would diminish to a point where the system's radio equipment cannot receive the end user's transmissions, or vice versa.
Conventional cellular communications systems address the handoff problem by monitoring and comparing signal strength. A currently used channel associated with one cell may be monitored and compared with a channel associated with another cell. This type of monitoring may be performed by a subscriber unit. Alternatively, a currently used channel may be monitored from locations in two different cells, and the results of this monitoring compared. This type of monitoring may be performed by system equipment located in diverse cells. Communications are then passed off to the cell with the stronger signal.
The conventional handoff technique may work adequately when the distances between subscriber units and system transceivers are relatively small, when the speeds of movement between cells and subscriber units are slow, and when handoffs are relatively evenly distributed in time. Such conditions are present for conventional terrestrial cellular systems in which cells do not significantly move with respect to the earth and the movement between cells and subscriber units results from subscriber movement in accordance with conventional modes of transportation. On the other hand, when system radio equipment is located on satellites orbiting the earth in moving orbits, these conditions are not present, and the conventional handoff techniques may be inadequate.
For example, orbiting satellites are located a relatively large distance from subscriber units, often on the order of several hundred kilometers. The smaller this distance, the greater the speed of the satellite relative to a particular postion on the earth. Speeds of over 20,000 km/hr are typical. This fast movement relative to a subscriber unit introduces widely and rapidly varying propagation delays and Doppler frequency offsets into signals transmitted between a satellite and a subscriber unit. The widely and rapidly varying propagation delays and Doppler frequency offsets make the acquisition and monitoring of signals a difficult and time consuming task, particularly when many different signals may be acquired. Moreover, the great speed of this movement may cause handoffs to occur much more frequently than in conventional cellular systems. For these types of systems not only does the cell pattern move, but its configuration changes with time; cell shutdown occurs so as to maintain channel separation. This leads to another form of imposed handoff.